Various methods of monitoring the physical states of transmission lines are adopted in optical transmission systems. For example, in a monitoring method using optical channel monitors (OCMs), the OCMs are dispersedly arranged on the transmission line or at nodes on the transmission line to monitor the physical state of each point on the transmission line from the results of measurement by the respective OCMs. With this method, the physical state of each point on the transmission line is capable of being monitored even when the transmission line has a multi-span configuration.
In a monitoring method using optical time domain reflectometers (OTDRs), optical pulses are transmitted on the transmission line to detect Rayleigh backscattered light and reflected light for the optical pulse from each point on the transmission line. With this method, the physical state of each point on the transmission line, for example, the transmission loss on the transmission line and the connection loss of, for example, fusion splicing, mechanical splicing, or connector connection are capable of being monitored based on the Rayleigh backscattered light and reflected light.
In a monitoring method using a coefficient used for an adaptive equalizer in main signal processing of a digital coherent receiver, the physical state of the transmission line is calculated using the coefficient. With this method, the physical state of the transmission line is capable of being monitored using the coefficient even when the transmission line has the multi-span configuration.
For example, refer to Japanese Laid-open Patent Publication No. 2012-124686, Japanese Laid-open Patent Publication No. 2006-295479, D.C. Kilper, R. Bach, D. J. Blumenthal, D. Einstein, T. Landolsi, L. Ostar, M. Preiss, and A. E. Willner, “Optical Performance Monitoring,” IEEE JLT22, 1, pp.294-304, 2004, and F. N. Hauske, M. Kuschnerov, B. Spinnler, and B. Lankl, “Optical Performance Monitoring in Digital Coherent Receivers,” IEEE JLT27, 16, pp.3623-3631, 2009.